In half a cup of warm water, add a teaspoon of sugar and yeast. Set aside in a warm place for it to froth up.Meanwhile in a stand mixer, add the flour, salt, sugar, yogurt, butter and knead until it resembles bread crumbs.Add in the yeast solution and knead (for about 8-10 minutes) adding as much water as needed to get a soft, smooth and elastic dough.In an oiled bowl, roll the dough, cover it and set aside in a warm place until it doubles in volume.Deflate the dough, divide into 8 equal portions. Knead each portion and roll into a tight ball. Place each of them in a greased baking pan spacing evenly.Cover and let them rise for another 30-40 minutes.Brush with milk and butter on the top and bake at 400 °F for about 20-30 minutes. Remove and sprinkle with powdered sugar on top and serve them warm.